This invention relates to a method for reducing, sintering and carburizing pre-alloyed, ferrous-base powder metal forging preforms.
The invention provides a method for making powder metal forging preforms of high strength ferrous-base alloys in which the times and temperatures for reducing, sintering and carburizing such preforms are substantially reduced. Additionally, the method produces forging preforms having an improved carbon gradient over forging preforms produced by current methods.
In the production of high strength, ferrous-base powder metal parts, pre-alloyed powder is cold pressed, sintered, and then forged. During sintering, oxides in the powder are reduced to acceptable levels. For parts which require case carburizing, it has been found advantageous to carburize the part prior to forging, preferably in the sintering furnace. Consequently, the sintering furnace produces an in-process product which may be described as a sintered and carburized forging preform. This forging preform is lubricated, heated to an appropriate forging temperature, and forged to a close tolerance configuration. After forging, the part is normally quenched and stress relieved.
A number of powder metal parts require a post forging machining operation, such as grinding. In such cases, it is necessary to insure that the carbon case introduced during carburizing has adequate depth and composition to permit the removal of material during machining while maintaining the specified surface carbon content. The carbon gradient produced by current powder metallurgy techniques has not been adequate for certain applications in which relatively high post-machining surface carbon is required.
In addition to the physical properties of forged powder metal parts, another important consideration is cost. The growth in the popularity of powder metal parts has been due, in many cases, to lower manufacturing costs as compared to the more conventionally manufactured products, such as machined-from-wrought parts. In other words, while it is recognized that powder metal and machine parts are, in many instances, comparable in physical properties, powder metal parts have been able to replace or compete with machined parts due to lower manufacturing costs. While lower manufacturing costs are generally the case, the difference in cost between powder metal and machined parts for higher strength applications, such as components for transmissions and axles, is smaller. This narrowing of the cost advantage is due, in part, to the more sophisticated process used to manufacture forged powder metal parts. Consequently, competition between forged powder metal parts and machined parts for these applications is severe. Improvements which lower the cost of manufacturing forged powder metal parts are quite important to enable these products successfully to compete with machined products.